It has been common to use a direct inward-dialing function in order to accommodate an outside line in an exchange and share this outside line among a plurality of extension terminals. In this case, an imaginary extension line is used so that a plurality of extension telephones can respond to an incoming call arriving at one dial-in number.
Specifically, an imaginary extension line is set as a destination of a dial-in incoming call. Then, this imaginary extension line is assigned to a function button of a plurality of extension telephones which are multi-function telephones. After that, when a dial-in incoming call occurs at this imaginary extension line, a plurality of extension telephones to which the imaginary extension line is assigned ring and imaginary extension buttons at the destinations corresponding to the imaginary extension line blink in red so as to notify that the dial-in incoming call has occurred. In this state, the user of the extension telephone can respond to the dial-in incoming call by pressing the blinking imaginary extension button.
Then, when a user of one of the plurality of extension telephones responds to the dial-in incoming call by pressing the imaginary extension button, the ringing in each of the extension telephones is stopped. Further, the light of the imaginary extension button of the extension telephone that has responded to the incoming call changes to green, thus indicating that the call is in progress. Meanwhile, the imaginary extension buttons of the other extension telephones are turned off.
In this state, when another dial-in incoming call occurs at the imaginary extension line, the extension telephones other than the one that has already responded to the dial-in incoming call ring. Further, the imaginary extension buttons blink in red so as to notify that the dial-in incoming call has occurred. A user of the extension telephone which in not in a busy state and whose imaginary extension button is blinking can respond to the dial-in incoming call by pressing the blinking imaginary extension button.
As described above, the imaginary extension line has such a mechanism that an extension telephone receives not only a call to a specific extension number of that extension telephone, but also an incoming call arriving through the imaginary extension line to which a plurality of extension telephones are assigned.
Because of this mechanism, even when all the extension telephones have responded to dial-in incoming calls and are all in a busy state, and there is no extension telephone that can respond to another incoming call, they still accept a new dial-in incoming call to the imaginary extension line.
In this case, since all the extension telephones are in the busy state, it is impossible to respond to the new dial-in incoming call.
Regarding this matter, Patent Literature 1 discloses a technique for preventing a dial-in incoming call that cannot be responded to as described above from occurring.
Specifically, in the technique disclosed in Patent Literature 1, the upper limit for the number of dial-in incoming calls is limited to the number of extension terminals. Then, when there is a dial-in incoming call by which the number of incoming calls exceeds the upper limit, it is indicated that the line is “busy” to that dial-in incoming call and the incoming call is not accepted. In this way, it is possible to prevent the situation in which a new dial-in incoming call is accepted even though there is no extension telephone that can respond to the incoming call at the moment, and hence it is impossible to respond to this dial-in incoming call.